This invention relates generally to apparatus for detecting a location of a movable member in a conduit and more particularly, but not by way of limitation, to magnetic detection apparatus for a reactor tube in which a viscous fluid is reacted and from which the reacted fluid is pumped.
In a fluid conducting system, movable objects, such as pigs or plugs, are used to separate volumes of fluids conveyed through a conduit of the system. It is sometimes necessary to detect where such a movable object is in the conduit. For example, it is sometimes necessary to determine that an object has reached an end of the conduit so that valves can be actuated to change the direction of flow through the conduit.
By way of a specific example, reference is made to the continuous reactor system described in U.S. patent application Ser. No. 615,440 filed 5-30-84 now U.S. Pat. No. 4,595,566. This continuous reactor system includes two cylindrical reactor vessels, specifically tubes, each of which contains a bidirectionally movable object separating a polymerizing fluid from a completed polymer. In the operation of this system, the objects (referred to as plugs or pigs, of which "pigs" will be used herein) are pumped from one end of their respective vessels to the other. When one of the pigs reaches the end of its respective tube, flow control valves must be actuated to reverse the flow in both vessels. If the flow from the reactor is to be continuous, this valve actuation and flow reversal needs to be automated. Such automation requires that the location of the pigs at the end of their respective vessels be automatically detected.
Several types of detector apparatus, by which a pig position or location indicating signal is generated, have been used or proposed to be used in association with objects moving through fluid conveying conduits. One of these types includes a mechanical switch having an actuator arm physically extending through the side wall of the conduit for mechanically engaging with the moving object. Such an object may be a spherical meter prover ball or the sides of a flexible pig, for example.
Such a mechanical switch extending into the environment of the conduit and relying upon suitable actuation by the pig has shortcomings. One of these is that the switch cannot be perfectly sealed so that fluid from the conduit leaks into the working components of the switch. When the fluid is a viscous polymer, for example, this leaked fluid gums up the switch components, thereby rendering the switch inoperative. Additionally, such a switch is sensitive to the pressure within the conduit since part of the switch is exposed within the conduit. Furthermore, when such a switch is used with a flexible pig, the flexible sides of the pig are sometimes not sufficiently rigid to consistently actuate the switch.
Another type of previously disclosed detector apparatus includes an electrically energized proximity switch that is placed outside the conduit, but in responsive association with a pressure balanced piston connected to the conduit so that an actuator arm from the piston responds to engagement by the pig. When the actuator arm is engaged so that the pressure balanced piston moves, the piston causes a change in an electrical characteristic, such as the reluctance, of the proximity switch circuit, thereby indicating the pig has reached the predetermined location.
One shortcoming of this second type of apparatus is that the piston is, similarly to the mechanical switch, rendered inoperative by viscous fluids, for example, within the conduit because such fluids enter the piston housing, which has openings for achieving the pressure balance. Additionally, this type requires an electronics package in that electrical power needs to be provided to the proximity switch for providing the voltage or current by which the electrical characteristic is reflected. Such an electronics package can be temperature sensitive so that its operation is not necessarily constant throughout different environments in which it might be used.
A third type of detector apparatus includes an electrical conductor coiled around the conduit through which a magnetic pig moves so that an electrical characteristic of the circuit of which the coil forms a part is changed in response to the pig moving through the portion of the conduit about which the coil is wrapped. Although this type does not have the mechanical problems associated with the mechanical portions of the forementioned switches, this type still requires an electronics package which must provide a relatively large current to operate the coil and which has been found to be temperature sensitive. Furthermore, the pig of this type of detector is made entirely of a magnetic material, which thereby prevents it from being made of another type of material which might be more suitable for withstanding deterioration brought about by the various fluids in which it might need to be used. This is particularly significant in a system wherein corrosive fluids are propelled or separated by pigs used in operations at an oil or gas well.
In view of the foregoing shortcomings, there is the need for an improved detector apparatus which is not adversely affected by the type of fluid used, or the pressures or temperatures existing, within the fluid conducting system. Such an apparatus should not require significant electronic packages so that external power is not needed at the conduit to initiate the detection. Such apparatus should also be one wherein the pig with which it is used need not be made of a special material to provide the activation, but rather can be made of a protective material suitable for the environment in which the pig will be used. The apparatus should also be constructed so that the interaction between the activating portion and the activated portion from which the location indicating signal is provided reliably occurs to consistently indicate the moving object has reached the predetermined location or position.